1. Field of the Invention
The present invention relates generally to pigments and foils. In particular, the present invention relates to robust multilayered pigment flakes and foils that have magnetic layers and which can also have optically variable characteristics, as well as pigment compositions that incorporate the multilayer magnetic pigment flakes.
2. Background Technology
Various pigments, colorants, and foils have been developed for a wide variety of applications. For example, magnetic pigments have been developed for use in applications such as decorative cookware, creating patterned surfaces, and security devices. Similarly, color shifting pigments have been developed for such uses as cosmetics, inks, coating materials, ornaments, ceramics, automobile paints, anti-counterfeiting hot stamps, and anti-counterfeiting inks for security documents and currency.
Color shifting pigments, colorants, and foils exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is changed. The color-shifting properties of pigments and foils can be controlled through proper design of optical thin films or orientation of the molecular species used to form the flake or foil coating structure. Desired effects can be achieved through the variation of parameters such as thickness of the layers forming the flakes and foils and the index of refraction of each layer. The changes in perceived color that occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials comprising the layers and wavelength dependent interference effects. The interference effects, which arise from the superposition of light waves that have undergone multiple reflections, are responsible for the shifts in color perceived with different angles. The reflection maxima changes in position and intensity, as the viewing angle changes, due to changing interference effects arising from light path length differences in the various layers of a material that are selectively enhanced at particular wavelengths.
Various approaches have been used to achieve such color shifting effects. For example, small multilayer flakes, typically composed of multiple layers of thin films, are dispersed throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object. Such flakes may optionally be overcoated to achieve desired colors and optical effects. Another approach is to encapsulate small metallic or silicatic substrates with varying layers and then disperse the encapsulated substrates throughout a medium such as paint or ink. Additionally, foils composed of multiple layers of thin films on a substrate material have been made.
One manner of producing a multilayer thin film structure is by formation on a flexible web material with a release layer thereon. The various layers are deposited on the web by methods well known in the art of forming thin coating structures, such as PVD, sputtering, or the like. The multilayer thin film structure is then removed from the web material as thin film color shifting flakes, which can be added to a polymeric medium such as various pigment vehicles for use as an ink or paint. In addition to the color shifting flakes, additives can be added to the inks or paints to obtain desired color shifting results.
Color shifting pigments or foils have been formed from a multilayer thin film structure that includes the same basic layers. These include an absorber layer(s), a dielectric layer(s), and optionally a reflector layer, in varying layer orders. The coatings have been formed to have a symmetrical multilayer thin film structure, such as:                absorber/dielectric/reflector/dielectric/absorber; or        absorber/dielectric/absorber.Coatings have also been formed to have an asymmetrical multilayer thin film structure, such as: absorber/dielectric/reflector.        
With regard to magnetic pigments, U.S. Pat. No. 4,838,648 to Phillips et al. (hereafter “Phillips '648”) discloses a thin film magnetic color shifting structure wherein the magnetic material can be used as a reflector or absorber layer. One disclosed magnetic material is a cobalt nickel alloy. Phillips '648 discloses flakes and foils with the following structures:                dyed superstrate/absorber/dielectric/magnetic layer/substrate;        dyed superstrate/absorber/dielectric/magnetic layer/dielectric/absorber/dyed superstrate; and        adhesive/magnetic layer/dielectric/absorber/releasable hardcoat/substrate.        
One attempt at incorporating a magnetic layer into a multilayer flake is disclosed in European Patent Publication EP 686675B1 to Schmid et al. (hereinafter “Schmid”), which describes laminar color shifting structures which include a magnetic layer between the dielectric layer and a central aluminum layer as follows:                oxide/absorber/dielectric/magnet/Al/magnet/dielectric/absorber/oxideThus, Schmid uses aluminum platelets and then coats these platelets with magnetic materials. However, the overlying magnetic material downgrades the reflective properties of the pigment because aluminum is the second brightest metal (after silver), and thus any magnetic material is less reflective. Further, Schmid starts with aluminum platelets generated from ballmilling, a method which is limited in terms of the layer smoothness that can be achieved.        
Accordingly, there is a need for improved magnetic pigment flakes and foils that overcome or avoid the deficiencies of prior flakes and foils.